The invention relates to a measuring device for sensing different gases and gas concentrations, which comprises a radiation source, a measuring channel having an optical path and a gas interaction path, and radiation detectors arranged along the measuring channel.
The invention also relates to a method for measuring gases and gas concentrations which uses the measuring device according to the invention.
There are two basic types of gas sensors: interacting and non-interacting gas sensors. In the first, a gas has to physically or chemically interact with a sensor element. In this case, the gas comes into contact with one or more components, for example, electrodes, electrolytes, or sensor surfaces of the gas sensor via, for example, oxidation, reduction, or physical adsorption. These interactions necessarily result in a change of the gas sensor, i.e., a change of sensor parameters in dependence on the interacting components of the sensor, for example, a change of the electrolytes interacting with the gas, whereby a regular calibration and finally the replacement of the gas sensor become necessary. The most frequently used interacting gas sensors are, for example, electrochemical sensors, solid-state sensors, and catalytic sensors.
Noninteracting gas sensors, also referred to as interaction-free, are optical gas sensors. For this purpose, only electromagnetic radiation comes into contact with the gas or interacts therewith, wherein a part of the radiation is absorbed by the gas molecules and the gas molecules thereupon change the excitation state thereof. However, the excited gas molecules return into the base state thereof due to collisions with other gas molecules or the sample chamber, and therefore the state of the gas does not change physically or chemically. The wavelength λ of the electromagnetic radiation extends in this case from the ultraviolet up into the far infrared spectral range (λ=0.2 μm to 20 μm). In this case, non-dispersive infrared (NDIR) gas sensors are the interaction-free gas sensors used most. In addition to the simple construction, they are distinguished above all by a high measurement resolution, a long service life, and good long-term stability. The method utilizes the excitation of energy states in molecules, i.e., the vibration excitation of molecular bonds, by infrared radiation. At these molecule-specific rotation and vibration frequencies, infrared radiation is absorbed. Because of the individual molecular structure, each molecule has very specific absorption bands in the infrared spectral range, whereby it can be unambiguously identified. In this case, the infrared spectral range λ=(2 . . . 20) μm is of technical interest, because the characteristic absorption bands of many compounds are in this spectral range.
The first practically usable NDIR gas sensor was developed in 1938 and is described in patent specification DE 730478. In this construction, the radiation originating from two radiation sources is periodically interrupted by a motor-driven aperture wheel and guided in two pipes separate from one another. The gas or gas mixture to be measured is located in one pipe and a reference gas is located in the other pipe. The radiation then enters two measuring chambers, which contain the gas that is to be detected as a receiver layer. These two measuring chambers are separated from one another gas-tight by a thin membrane. Gas-tight means that no gas is exchanged between the chambers. The membrane forms, with a counter plate arranged in an insulated manner, an electrical capacitor, the capacitance of which can be read off using a measuring instrument. The absorption of the infrared radiation by the gas is therefore detected as a pressure difference by means of a very sensitive microphone. This NDIR gas sensor, which is known as a photoacoustic gas measuring cell, has the significant disadvantages of its structural size and the mechanical susceptibility to vibrations and shocks.
A technology which enabled significantly smaller and more robust NDIR gas measuring devices was finally available with the development of nondispersive, very narrowband optical filters. These so-called interference filters use the effect of interference in order to filter electromagnetic radiation in dependence on the frequency and/or wavelength. In an embodiment as a bandpass filter, a specific wavelength band is transmitted, while shorter and longer wavelengths are reflected or absorbed. The transmission maximum is defined as the center wavelength (CWL) of the bandpass filter. The bandwidth of the filter is specified by the full width at half maximum (FWHM), i.e., the difference between the two argument values for which the function values have dropped to half of the maximum. The transmission spectrum of the bandpass interference filter is finally selected in such a way that it corresponds to a characteristic absorption band of the gas to be measured. The absorption of the infrared radiation by the measured gas is measured in this case using a very sensitive radiation detector, which is arranged behind the bandpass interference filter.
The radiation attenuation caused by the gas as a result of radiation absorption is finally a measure of the gas concentration. The radiation intensity IM of the measurement wavelength changes in this case as a function of the gas concentration c according to the Lambert-Beer law:IM=I0·eα·c·lwherein α denotes the gas-specific absorption coefficient, 1 denotes the absorption path length, and I0 denotes the base intensity of the radiation, i.e., in the absence of the measured gas (c=0).
A simple NDIR gas sensor therefore consists of an infrared radiation source, a measuring chamber (cuvette), in which the gas or gas mixture to be measured is located, and also an infrared detector having a bandpass interference filter, the transmission spectrum of which corresponds to the absorption band of the gas to be measured (FIG. 1). These components are installed along an optical axis. Such a construction is described, for example, in the documents DE 10221708 B4 and DE 10013374 A1. In general a thermal radiator which can be electrically modulated is used, which emits electromagnetic radiation having a continuous spectrum as a result of its temperature, in which all wavelengths of the spectral range of technical interest λ=(2 . . . 20) μm are included. A broad palette having sufficient signal-to-noise ratio and low price is available as the infrared detector, for example, thermopile sensors and pyroelectric detectors.
Modern NDIR gas sensors, as are known, for example, from the documents DE 10 2008 005 572 B4, DE 20 2005 010 475 U1, DE 102 21 708 B4, and DE 296 02 282 U1, are usually operated according to the so-called two-frequency method (FIG. 2). In this case, in addition to the measurement at a measurement wavelength adapted to the measured gas, a measurement is additionally carried out at a second wavelength, the so-called reference wavelength, which lies in a spectral range in which no absorption takes place due to other gases present in the gas mixture or in the surroundings. For this purpose, two infrared detectors arranged in the beam path having different bandpass interference filters are necessary. By means of quotient calculation of the two detector signals, a substantial stability improvement is achieved in this case, whereby, for example, signal changes as a result of intensity drifts of the radiation source or dirt deposits in the measuring chamber can be compensated for. However, the required allocation of the radiation flux emitted by the radiation source onto the two infrared detectors is disadvantageous, whereby the radiation intensity at the infrared detector and thus the detection limit of the gas sensor are reduced. FIG. 2 shows the two-frequency method according to the prior art and the required allocation of the radiation emitted by the radiation source S onto the detectors D1 and D2.
However, only one gas can be measured using the above-mentioned measuring methods and measuring devices suitable for this purpose. In many gas-analytic applications, for example, in the case of an exhaust gas, flue gas, or anesthesia gas measurement, however, it is necessary to sense multiple gases simultaneously and determine the concentrations thereof in the gas mixture.
A so-called multispectral detector for NDIR gas sensors is presented in US 2012/0235038 A1, which has a plurality of detector elements having bandpass interference filters. This enables a simple construction of the gas sensor in accordance with the above-described two-frequency method for a simultaneous measurement of multiple gases. Similar arrangements are also known from the documents DE 34 06 175 A1, DE 41 33 481 A1, and DE 101 40 998 C2. Significant disadvantages of such NDIR gas sensors having multispectral detectors are a constant absorption path or cuvette length for all spectral channels and the distribution of the radiation intensity onto the individual detector elements. Thus, for example, in the case of a four-channel detector, in the ideal case only 25% of the incident radiation intensity is available per detector element for signal generation. In reality, it is usually less than 10%. The detection limit of the gas sensor is therefore greatly reduced in the case of all gases to be measured.
A constant absorption path length for all spectral channels limits, on the one hand, the measurement range and, on the other hand, the detection limit of the gas sensor. This results from the fact that firstly every gas has a gas-specific absorption coefficient, secondly gases are usually present in different concentrations in a gas mixture, and thirdly gases have different toxicities, because of which different limit values apply, which in turn require different measurement resolutions. Thus, for example, the respiratory toxins contained in the flue gas of an oil firing, carbon dioxide (CO2), sulfur dioxide (SO2), nitrogen monoxide (NO), and carbon monoxide (CO), are present in the following concentrations:                (125,000 . . . 140,000) ppm CO2,        (180 . . . 220) ppm SO2,        (80 . . . 150) ppm CO, and        (50 . . . 100) ppm NO.        
The maximum workplace concentrations (MAK values) of these flue gas components can be taken from the technical rules for hazardous substances (TRGS 900) and are:                5000 ppm for CO2,        0.5 ppm for SO2,        25 ppm for NO, and        30 ppm for CO.        
The radiation attenuation caused by the gas as a result of radiation absorption according to the Lambert-Beer law is thus individual for each gas and therefore an individual absorption path length is reasonable so that an accurate concentration determination can be carried out. Otherwise, a compromise always has to be made with respect to measurement range and measurement resolution.
A gas sensor device for sensing the gas concentrations in a complex gas mixture is proposed in DE 19604167 A1, in which the individual radiation detectors are arranged rotationally-symmetrically about a radiation source, wherein the distance to the radiation source and thus the absorption path length can vary. The allocation of the emitted radiation flux onto a plurality of radiation detectors remains a significant disadvantage of this arrangement, whereby only a very small fraction of the radiation intensity arrives at the detectors and the detection limit of the gas sensor device is greatly reduced. This also applies to the arrangement described in U.S. Pat. No. 5,222,389 A, in which the individual radiation detectors are arranged along the measuring chamber to implement different absorption path lengths. However, a substantial disadvantage therein is also that only a fraction of the measurement radiation is always incident on the detectors, which is moreover dependent on the respective reflection on the measuring chamber wall.
Furthermore, an NDIR gas sensor having only one radiation detector, in which the selection of the measurement wavelength is performed by a filter wheel, is known from KR 1020100052691 A. This filter wheel can be equipped with matching bandpass interference filters in accordance with the gases to be measured, whereby a variety of gases may be identified using a simple construction. However, this can only take place sequentially. A simultaneous sensing of the concentration of different gases in a gas mixture is not possible. Moreover, the absorption path length for every gas is equal and the arrangement may only be miniaturized poorly.
For an improvement of the detection limit of optical gas sensors, it is necessary to focus the highest possible radiation intensities onto the radiation detector. Furthermore, an individual absorption path length is to be provided for every measured gas, so that an optimum determination of the individual gas concentrations in a complex gas mixture can take place and the structural size of the gas sensor can be kept minimal. For many applications, a simultaneous determination of the components in a gas mixture is moreover required.